International Court of Justice Advisory Opinion finds States have binding legal obligations to address climate change

On 23 July 2025, the International Court of Justice ("ICJ") issued an advisory opinion, finding that States have binding legal obligations under international law to address climate change and protect the environment. 

While the ICJ’s opinion is not legally binding, it does provide an authoritative interpretation of legally binding obligations and carries substantial legal and moral weight, so may well influence States’ policies (including in relation to certain sectors, such as fossil fuels).

Key takeaways

• States have an obligation to protect the climate system and other parts of the environment from the harmful effect of greenhouse gas ("GHG") emissions, including by regulating the activities of private actors such as businesses. 
• These duties arise not just from climate change treaties, but from a wide range of international law (including customary international law, human rights frameworks and other environmental treaties). 
• The ICJ considered in detail States’ obligations under the Paris Agreement and how they are reflected in the obligation to prepare, communicate and maintain Nationally Determined Contributions ("NDCs") – particularly notable given the expectation that States submit updated NDCs at COP 30 in Brazil in November 2025. 
• Breach of their obligations constitutes an internationally wrongful act for which States may be liable for full reparation to injured States, including through restitution, compensation, and satisfaction. 
• The ICJ also established that the primary temperature goal under the Paris Agreement is to maintain global warming below 1.5 degrees Celsius, and that developed countries must take the lead in combating climate change and provide financial assistance to developing countries for both climate mitigation and adaptation activities.

Role of the International Court of Justice and putting the opinion into context

The ICJ is the main judicial arm of the United Nations, widely known as the “world court”, and was established in 1945 as a way to settle disputes between UN Member States. 

Unlike the Court of Justice of the European Union, the ICJ does not function as an overarching ‘supreme court’ to which national courts can refer cases – it can only hear disputes between countries and where requested by one or more States. The ICJ can also provide advisory opinions on questions that have been referred to it by other authorised UN entities. 

This opinion is the third of four advisory proceedings at international courts, clarifying state obligations in relation to climate change, and the second opinion issued in four weeks:

• On 3 July 2025, the Inter-American Court of Human Rights opined that States have a duty to mitigate and address environmental harms threatening human rights (such as climate change), including by adopting national laws, policies and actions (see our blog post).
• In May 2024, the International Tribunal for the Law of the Sea ("ITLOS") issued an advisory opinion confirming that all anthropogenic GHG constitute marine pollution and clarifying that States must take all measures to prevent, reduce and control emissions and to protect and preserve the marine environment against the impact of climate change.
• In May this year, a petition was filed with the African Court on Human and Peoples’ Rights for an advisory opinion on the obligations of African Union member states in the context of climate change. The timing for when this final opinion will be issued remains unclear.

While these opinions are not legally binding, they are authoritative interpretations of legally binding obligations and carry substantial legal and moral weight. This contrasts with court decisions on similar topics, including the 2024 European Court of Human Rights’ ruling that States that are Parties to the European Convention of Human Rights have an obligation under Article 8 to provide protection against adverse effects on human health from harmful effects and risks caused by climate change, which is binding on all States that are Parties to that Convention (see our blog post).

Scope of the ICJ’s opinion

On 29 March 2023, the UN General Assembly adopted a resolution asking the ICJ to opine on the obligations of State in respect of climate change under international law (resolution 77/276). 

The General Assembly requested the ICJ to consider two questions:

1. the obligations of States under international law to ensure the protection of the climate system and other parts of the environment from anthropogenic GHG emissions; and
2. the legal consequences for States where they, through their acts and omissions, cause significant harm to the climate system and other parts of the environment, both with respect to States (in particular, small island developing States) and to present and future generations. 

The ICJ noted it would not limit its inquiry to State conduct that, itself, directly resulted in GHG emissions. Rather, it found that the “material scope” of its inquiry encompassed the full range of human activities that contribute to climate change due to their GHG emissions, including consumption and production activities. 

The ICJ also identified the most relevant applicable law governing these questions, noting in particular the United Nations Framework Convention on Climate Change ("UNFCCC”), Kyoto Protocol, and the Paris Agreement as the principal legal instruments regulating the international response to climate change. It also considered, in turn, customary international law, international human rights law, and the extent to which it should take into account other principles of international law (including principles of common but differentiated responsibility and respective capabilities, equity and intergenerational equity, and the precautionary approach). 

A. Obligations of States

Obligations of States under international climate change treaties

The ICJ emphasised that the UNFCCC acts as the foundational treaty to address climate change, with the Kyoto Protocol operationalising certain obligations under the UNFCCC by committing certain developed country parties to limit and reduce GHG emissions according to agreed individual targets. The Paris Agreement similarly provides specifics of the general obligations in the UNFCCC by requiring State parties to aim to reach global peaking of GHG emissions as soon as possible. The ICJ also clarified that, while the Paris Agreement provides for limited global warming to well below 2 degrees Celsius above pre-industrial levels as a goal and 1.5 degrees Celsius as an additional effort, it considered that 1.5 degrees Celsius has become the scientifically based consensus target and therefore State parties’ agreed primary temperature goal. 

Many obligations identified by the ICJ as arising from these climate change treaties are well-known (e.g. the obligation to develop, periodically update, and make available national inventories of GHG emissions and carbon sinks and to plan for and implement climate change adaptation measures under the UNFCCC, and the obligation to develop and set NDCs under the Paris Agreement), having been part of the international legal landscape for ten or more years. However, the ICJ’s opinion makes clear that these constitute legally binding obligations on States, breach of which constitutes an internationally wrongful act for which they may be liable to other, injured State parties. 

In considering these State duties, the ICJ differentiates between obligations of result and obligations of conduct, noting that to comply with an obligation of conduct, States must use all means at their disposal to bring about the intended objective and are not considered to have acted wrongfully if they have done so but ultimately failed to achieve their objective. However, for an obligation of result, States must adopt measures such that they are able to achieve the goal required (i.e. simply taking the measures is not, in itself, sufficient to discharge their obligation). 

The ICJ also emphasised international law principles of differentiated responsibility and differing capacity, reiterating that developed countries (e.g. in the context of the UNFCCC, those countries listed in Annex I to the instrument as ‘industrialised’ or ‘economies in transition’) are obliged not only to take the lead in fulfilling their duties under climate change treaties, but also to provide financial assistance to developing countries for both climate change mitigation and adaptation activities. 

Obligations of States under customary and other international law

The ICJ emphasised that the scope of existing State obligations under customary international law to prevent significant harm to the environment and to cooperate for the protection of the environment includes harm to the climate system and harm to the environment arising out of climate change.

States are obliged to act with due diligence to fulfil their duties which, in this context, means using all means at their disposal to prevent activities from being carried out in their jurisdiction or control which cause such harm, taking into account States’ differentiated responsibility and respective capabilities. Such appropriate rules and measures could include (but are not limited to) national regulatory GHG mitigation mechanisms.

The ICJ noted that, presently, it considered compliance in full and good faith by a State with its duties under the UNFCCC, Kyoto Protocol and Paris Agreement (as interpreted by the ICJ in this opinion) suggests substantial compliance with its duties under customary international law. 

The ICJ also noted that States’ duties under particular environmental treaties (e.g. the Ozone Layer Convention, the Montreal Protocol, the Convention on Biological Diversity, and the United Nations Convention to Combat Desertification) each play a role in protecting the climate system and are therefore also relevant to the consideration of State duties to combat climate change. 

Obligations of States under international human rights law

The ICJ stated that adverse effects of climate change including, among others, the impact on the health and livelihoods of individuals through events such as sea level rises, drought, desertification, and natural disasters may significantly impair the enjoyment of certain human rights (including the right to health, life, privacy, family, and home). The ICJ noted that the right to a clean, healthy and sustainable environment is a precondition for enjoyment of many of these, and other, human rights. 

In addition, the ICJ highlighted that conditions resulting from climate change which are likely to endanger people’s lives may lead them to seek safety in another country or prevent them from returning to their own (i.e. ‘climate refugees’). The ICJ emphasises States’ obligations of non-refoulement, preventing countries from returning refugees or asylum seekers to a place where they may face real risk of irreparable harm to their right to life. 

B. Legal consequences for States of breaching their international obligations

The ICJ recognised that climate change is extremely complex and multifaceted and gives rise to particular issues relating to attribution and causation and challenges in relation to the establishment of factual and legal causation of damage for which reparation is claimed. 

The ICJ clarified that causation is a concept that plays a role in determining reparation, not responsibility. It notes that causation involves two distinct elements: first, whether a climactic event or trend can be attributed to anthropogenic climate change; and second, to what extent damage caused by climate change can be attributed to a particular State or group of States. The ICJ found that failures of a State to take appropriate action to protect the climate system from GHG emissions (including through fossil fuel production or consumption, granting of fossil fuel exploration licenses, or providing fossil fuel subsidies, or failures to take necessary regulatory and legislative measures) may constitute an internationally wrongful act attributable to that State. 

States responsible for an internationally wrongful act have a duty to cease that act. This duty may require States to use all means at their disposal to reduce their GHG emissions and take other measures that ensures compliance with their obligations and could extent to providing appropriate assurances and guarantees of non-repetition. In addition, they are under a duty to make reparations to injured States or, in the case of a breach under international human rights law, injured individuals, which includes restitution, compensation, and/or satisfaction. In this context, it is important to note the ICJ’s view that loss of a State’s territory and displacement of its population (e.g. through rising sea levels) does not necessarily entail the loss of its statehood, leaving open the potential for small island States, who are disproportionately affected by climate change, to bring inter-State claims for reparation even in the even their territory is lost. 

What’s next?

The ICJ’s opinion has attracted significant public attention. However, its impacts on State actions and regulatory frameworks remain to be seen as, in many jurisdictions, this will not change the law as it currently stands (although it does provide an authoritative interpretation of State obligations). 

It is likely that it (along with the ITLOS and Inter-American Court of Human Rights opinions referred to above) will be used as building blocks for future climate litigation, in particular in domestic litigation challenging a State’s progress towards its international obligations. 

For more detail on recent climate trends, see our blog post on the 2025 report by the Grantham Institute on Climate Change and the Environment.